I've played both union and league, even played at academy level league. They are both great, in my opinion union has much more physical scrums. League there isn't much effort in the scrums, unlike union. Also I like line outs, gets a chance to play territory at times, but this can slow the game right down to an unbearable speed. But if you take line outs and long scrums out of the game like league has its a lot more physical as it?s faster with more contact due to the amount of tackles.
League is much faster and the fitness levels are much higher and you?re right though it is the more interesting sport for spectators. League seems to finally be expanding into different areas with new teams like Celtic, Catalan and Toulouse all recently developing the game. Also I hear rumours that Scotland are in the midst of developing a team (Edinburgh I reckon) to play in the SL. The Magic weekend seemed like a success.
I like both but do prefer league over union.